Controlling the rate of diamine curing agents in polyurethane cements

ABSTRACT

A method for preparing a polyurethane which has good long term flexibility, solvent resistance and resistance to hydrolysis, said method comprising mixing three components just prior to curing said components comprising a prepolymer solution, a cement solution and a crosslinking solution, said crosslinking solution comprising a solvent, an aromatic diamine and the ketimine of an aliphatic diamine.

TECHNICAL FIELD

This invention relates to a process for carefully controlling the rateof cure of polyurethane cements by using as a crosslinking component orcuring agent a mixture of an aromatic diamine and a ketimine which isthe reaction product of an aliphatic diamine and a carbonyl compoundsuch as a ketone.

BACKGROUND ART

The process of this invention is an improvement over the disclosures ofU.S. Pat. Nos. 4,247,678 and 4,496,707. In particular, this inventionrelates to an improved process for preparing polyurethane products ofthe type described in U.S. Pat. No. 4,496,707, the improvementcomprising the use of a mixture of an aromatic diamine and an aliphaticdiamine ketimine curing agent.

It is known to use aromatic diamines as curing agents for polyurethanecements and that the use of aliphatic diamines for this purpose isundesirable because exceedingly fast rates of cure usually occur whenthe aliphatic diamines are used. Although aromatic diamines arepreferred as curing agents for polyurethane cements, they give a slowrate of cure. Ketimines of aromatic diamines have been described ascomponents of certain flexible sealants and corrosion inhibitive primersused to protect the exterior skin and fastener patterns of highperformance aircraft at low temperatures. These sealants and primerswhich are composed of an epoxy resin, pigments including corrosioninhibitor, polyurethane elastomer, ketimine curing agent, and solventsfor sprayability are disclosed in U.S. Pat. No. 4,101,497.

Bicyclic aromatic diamines, such as 4,4'-methylene-bis(2-chloraniline)(MOCA), 4,4'-methylenedianiline (MDA), and 4,4'-diaminodiphenylsulfone(SDA) can be used as curing agents for polyurethanes. It would be highlydesirable to minimize the use of the bicyclic aromatic diamines and touse in their place to the extent possible, the aliphatic diamines suchas metaxylene diamine (MXDA), isophorone diamine (IPDA),1,3-bis(aminomethyl)cyclohexane (1,3 BAC) and others which have lessapparent carcinogenic potential than the aromatic diamines.

Unfortunately, aliphatic diamines cause much faster rates of cure inpolyurethane cements than the aromatic diamines do. This undesirableproperty has seriously limited the use of aliphatic diamines as curingagents in polyurethane cements.

DISCLOSURE OF INVENTION

I have found that the rate of cure of polyurethane cements can becontrolled within carefully predetermined limits by using a mixture ofan aromatic diamine and a ketimine of an aliphatic diamine as the curingagent in the polyurethane cement. The ketimine of the aliphatic diamineis readily produced by the Schiff's base reaction of an aliphaticdiamine and a carbonyl compound as follows: ##STR1## wherein R is analkyl or aralkyl group, R' is hydrogen or a hydrocarbon group and R" isa hydrocarbon group. In the foregoing reversible reaction, the waterformed is removed from the reaction by some known means such as the useof molecular sieves manufactured by Union Carbide Company, silica gel orcalcium sulfate to produce the stable ketimine. In practice, thesolution of the aliphatic diamine in an excess of ketone plus dryingagent is allowed to stand for at least 24 hours at ambient temperaturebefore use in the curing compound.

Aliphatic diamines useful in the practice of this invention includemetaxylene diamine (MDA), isophorone diamine (IPDA),1,3-bis(aminomethyl)cyclohexane (1,3 BAC), ethylene diamine (EDA),methylene bis(cyclohexylamine)(H₁₂ MDA) and metatetramethylenexylenediamine (MTMXDA), and the like.

Carbonyl compounds useful in the preparation of ketimines used in thisinvention include ketones, such as methyl ethyl ketone, methyl isobutylketone, acetone, and the like.

I have found that the use of mixtures of an aromatic diamine and analiphatic diamine, per se, will not modify the cure rate of polyurethanecements. In such cases, the fast rate caused by the aliphatic diaminetakes over and the modifying influence one might expect from thearomatic diamine is not observed. It is only when the aliphatic diamineis in the form of its ketimine that the cure rate can be controlled byuse of the blends of curing agents in accordance with this invention.Preferred blends of this invention are composed of solutions of anaromatic diamine and the ketimine of an aliphatic diamine.

This invention will allow the close control of the cure rate forpolyurethane cements without adversely affecting the fuel and hydrolysisresistance of the cured product.

The cure rate for a given polyurethane cement formulation is set by therate associated with a given curing agent. Slight variations in curerate for a given curing agent can be achieved by using more curing agentto increase the rate or by using plasticizers to decrease the rate, butboth of these methods for slightly altering rate are detrimental to thefuel resistance and hydrolysis resistance of the cured polyurethane fueltank material. Ambient conditions such as temperature and humidity canalso have some minor effect on cure rate of polyurethane cements butthey are not controllable in the less than ideal environment found inmost fuel tank production facilities. Long term retention of goodphysical properties in flexible fuel tank bladder applications isimportant because the bladder often must be removed and later replacedafter repairs or alterations are made to the fuel tank. Such proceduresare not possible or are extremely difficult if the fuel tank linerbecomes too stiff on aging. The fuel tanks prepared by the process ofthis invention retain flexibility and other desirable properties such asfuel and hydrolysis resistance over long periods of time.

Thus, the primary purpose of this invention is to provide a means forthe accurate control of the cure rate of polyurethane cements by thecareful selection of a combination of an aromatic diamine and a ketimineof an aliphatic diamine as curing agent for said cements.

In general, a process for preparing a urethane formulation, comprisingthe steps of: separately preparing a polymer component, said polymercomponent comprising a urethane prepolymer and a urethane solvent;separately preparing a curing component, said curing component beingcomposed of a mixture of an aliphatic diamine ketimine and an aromaticdiamine curing agent and a solvent; separately preparing a cementcomponent, said cement component comprising an epoxy having solventleaching resistance, and a solvent; a thixotropic compound, saidthixotropic compound located in said curing component, or in said cementcomponent, or in both; the total amount of solids in said overallformulation ranging from about 20 percent to about 60 percent by weightbased upon the total amount of said solvents and said solids in saidoverall formulation.

In general, a polyurethane, said polyurethane made by separatelypreparing a polymer component, a curing component, and a cementcomponent; said polymer component comprising a urethane prepolymer and aurethane solvent, said curing component being composed of a mixture ofan aromatic diamine and an aliphatic diamine ketimine curing agent and acuring agent solvent, said cement component comprising an epoxy havingsolvent leaching resistance, and a solvent; a thixotropic compound, saidthixotropic compound located in said curing component, or in said cementcomponent, or in both; said urethane produced by mixing said threecomponents together and curing at a temperature of from about ambient toabout 180° F.

In the curing agent mixture the proportion of the aromatic diamine tothe ketimine of the aliphatic diamine (expressed in terms of aliphaticdiamine) should fall in the range of from 10 to 90, and preferably fromabout 10 to 50 percent by weight, respectively.

BEST MODE FOR CARRYING OUT THE INVENTION

According to the present invention, a polyurethane is made by preparingthree separate components. Prior to application or upon application, thecomponents are mixed together and applied to a substrate or item. Eachof the separate components has good stability and hence the paint orcoating has a good shelf life. The urethane, when applied, has very goodsealant properties, is free from agglomerates, and is resistant tofuels.

One of the components is a urethane component in which a urethaneprepolymer is dissolved in a solvent. A curing agent component iscomposed of a mixture of an aromatic diamine and an aliphatic diamineketimine curing agent dissolved in a solvent. The last component is acement solution which generally contains an epoxy, an optional levelingagent, and a solvent. A thixotropic compound can be contained in eitherthe cement component, the cure component, or in both, generallydepending upon the pressure limitations of the spraying apparatus to beused for dispensing the formulation. When combined, the components forma polyurethane formulation generally having an overall solids content offrom about 20 percent to about 60 percent, preferably from about 40percent to about 50 percent, and optionally about 42 to 43 percent byweight based upon the total weight of the polyurethane formulationcontaining all three components. The solids content is generallyimportant in that too much solvent results in loss of an adhesive typefilm because the polyurethane, when used as a coating or paint, wouldtend to run whereas the use of too little solvent would result in aporous layer.

The polymer component comprises a polyester urethane prepolymer such asthat set forth in U.S. Pat. No. 4,247,687 which is hereby incorporatedby reference. Essentially, the urethane prepolymer is made by reactingan aliphatic or alicyclic polyisocyanate, preferably an alkyl or acycloalkyl polyisocyanate, with a polyester derived from the reaction ofa glycol and a mixture of or co-condensation of an aliphaticdicarboxylic acid, preferably an alkyl dicarboxylic acid, and/or anaromatic dicarboxylic acid. The aliphatic or alkyl polyisocyanate cancontain from 3 to 12 or more carbon atoms, with from 6 to 10 beingpreferred, and the alicyclic or cycloalkyl polyisocyanate can have from5 to 25 or more carbon atoms, with from 8 to 15 being preferred.Generally, diisocyanates are preferred. An example of a preferredpolyisocyanate is methylenebis(4-cyclohexylisocyanate), sold under thetrademark Desmodur W, by Mobay Chemical Corporation.

Suitable polyols include those having a molecular weight of 500 or less,or desirably 200 or less. Preferably, the polyol is a glycol containingat least 2 carbon atoms as from 2 to about 10 carbon atoms, with fromabout 4 to about 6 being desired. Examples of specific glycols include1,6-hexane diol, 1,4-butane diol, and ethylene glycol. The varioushexane diols are preferred. Aliphatic or alkyl dicarboxylic acidsgenerally contain from about 2 to about 12 carbon atoms and preferablyfrom about 6 to 9 carbon atoms. Specific examples include glutaric acid,adipic acid, and pimelic acid, with azelaic acid being preferred.Similarly, the aromatic dicarboxylic acids can contain from 8 to about12 carbon atoms with 8 carbon atoms being preferred. Examples of suchacids include terephthalic acid, isophthalic acid, and naphthalenedicarboxylic acid, and the like, with isophthalic acid being preferred.The ratio of the aliphatic dicarboxylic acids to the aromatic acids ineither the copolymerization reaction or as a physical mixture variesfrom 10 to 90 percent by weight. The polyester is formed by the reactionof the acids and the glycol according to any conventional process andgenerally has a molecular weight of from about 500 to about 4,000, withfrom about 1,000 to about 3,000 being preferred. The ratio of theequivalent amount of isocyanate used to the equivalent amount of hydroxyend groups in the polyester ranges from about 1.5 to about 3.0 andpreferably from about 1.8 to about 2.2.

Any conventional solvent can be utilized to dissolve the urethaneprepolymer. Specific examples include aromatic compounds having from 6to 10 carbon atoms and aliphatic, preferably alkyl substitutedcompounds, having from 3 to 8 carbon atoms. Examples of aromaticsolvents include xylene, toluene, benzene, and the like, whereasexamples of aliphatic compounds include methyl ethyl ketone, methylisobutyl ketone, and the like. An amount of solvent is utilized suchthat the amount of urethane prepolymer solids in the polymer componentranges from about 50 to about 75 percent, desirably from about 60 toabout 65 percent, and optimally from about 63 to about 65 percent byweight.

The cure component is composed of a mixture of an aromatic diamine andan aliphatic diamine ketimine curing agent in a solvent. The preferredaliphatic diamines include alkyl diamines containing from 2 to 10 carbonatoms, cycloalkyl diamines containing from 4 to 20 carbon atoms, andalkaryl diamines containing from 8 to 20 carbon atoms. The preferredaromatic diamines useful in the curing agent mixtures of this inventioninclude monocyclic aryl diamines containing from 6 to 12 carbon atomsand bicyclic aryl diamines containing from 12 to 20 carbon atoms.

The curing agent mixture is dissolved in any conventional solvent suchas the same solvents utilized with the urethane polymer, for example, aketone or an aliphatic hydrocarbon solvent. Examples of specificpreferred solvents include methyl ethyl ketone, methyl isobutyl ketone,and the like. The amount of solvent is such that the amount of aromaticdiamine and aliphatic diamine ketimine contained therein generallyranges from about 5 to about 30 percent, and optimally about 13 to about15 percent by weight. Of course, it should be understood that the amountof solvent in the cure component, as in the polymer component, can bevaried over a wide range so long as the total solvent or percent solidsin the final paint, when all three components are combined, is as setforth above. The amount of diamine (the aliphatic part of which is inthe form of its ketimine) when combined with the polymer component issuch that the equivalent ratio of diamine to diisocyanate ranges fromabout 0.8 to about 1.2 and desirably from about 0.9 to about 0.95, andoptimally about 0.93. Excessive amounts of diamine will result inreduced hydrolysis resistance whereas too small amounts will result inunsuitable solvent or jet fuel resistance and a reduced cure rate.

Inasmuch as the overall polyurethane formulation is generally based upon100 parts by weight of the urethane prepolymer, the amount of theurethane solvent can be readily calculated therefrom. Similarly, theamount of the mixture of aromatic diamine and the aliphatic diamineketimine can readily be calculated by the equivalent ratio ofdiisocyanate to the combined diamines. Hence, the amount of diamine anddiamine ketimine solvent also readily can be determined.

The third component, that is the cement component, generally comprises aleveling agent, an epoxy compound, and a solvent. Moreover, either thecement component, the cure component, or both can contain a thixotropicagent. Should the polyurethane formulation be sprayed at a relativelylow pressure, for example, at 125 psi or less, at 100 psi or less, oreven at 50 psi or less, the thixotropic agent is desirably blended inthe cure component when a pigment is utilized. Otherwise at lowpressures when a pigment is used in a cement component, agglomerations,globules, etc., usually form due to contact of the thixotropic agent andthat pigment. Such agglomerations can often jam up the spray gun and/orresult in a rough coating surface. Such a problem can be eliminated ifthe formulation is strained as through an 80 to 100 mesh screen.However, this process is not practical in large batch operations sincethe straining must be performed on cement that contains the mixture ofaromatic diamine and aliphatic diamine ketimine curing agent. Such couldresult in permanent clogging of the strainer with reacted urethane. Thethixotropic agent is important in that it is utilized to generally keepthe polyurethane cement from running. The amount of thixotropic agentgenerally ranges from about 2 parts to about 8 parts by weight basedupon 100 parts by weight of urethane prepolymer in the overallpolyurethane formulation, and desirably from about 3 to about 6 parts.Generally, any conventional thixotropic compound can be used. Suitablethixotropic agents include silicon dioxide, Cab-O-Sil (Cabot Corp.)surface modified aluminum silicate, manufactured by Georgia KaolinCompany and sold under the brand name "Kaophile #2," and the like. Thesecompounds generally exist as very fine particles so that they can beincorporated in the overall polyurethane formulation and sprayed as froma spray gun.

Although a leveling agent is generally utilized such that the paintsurface is smooth, it need not always be included in the paintformulation. Generally, any conventional leveling agent can be utilizedsuch as cellulose acetate butyrate and "Modaflow," manufactured byMonsanto Industrial Chemicals Company. The amount of the leveling agentgenerally varies from about 0.5 parts by weight to about 3 parts byweight, based upon 100 total parts by weight of urethane in theformulation. A more desired range is from about 0.5 parts to about 1.25parts by weight. Inasmuch as the leveling agent tends to reduce surfacetension and hence tends to counteract a thixotropic agent, high amountsthereof are not utilized.

With regard to the epoxy compound, a nonleachable type is preferred suchthat it does not leach out in the presence of solvents or fuels residingupon the final paint formulation. An example of one such specific epoxyresin in Epon 1001, manufactured by the Shell Chemical Company.Generally, this epoxy is 4,4'-isopropylidenediphenol-epichlorohydrin.Another suitable epoxy is D.E.R. 732, manufactured by the Dow ChemicalCompany. This epoxy is an epichlorohydrin-polyglycol reaction product.The amount of epoxy generally ranges from about 3 to about 7 parts byweight with from about 5 to about 6 being preferred, based upon 100parts by weight of urethane polymer in the overall final polyurethaneformulation.

Generally, any colorant such as a pigment or dye can be utilized in thecement component to impart a desired color. The amount can range fromvery small, e.g., from about 10 to about 15 parts by weight. Examples ofvarious pigments include carbon black, titanium dioxide, chromic oxide,and the like. Of course, many other pigments in various amounts can beutilized to achieve a desired color or hue.

The amount of solvent in the cement component can vary over a wide rangeand exists in such amounts that when the three components are blendedtogether, the total amount of solids in the overall paint formulationranges from about 20 percent to about 60 percent by weight. Typically,the amount of solvent existing in the cement component ranges from about25 parts by weight to about 250 parts by weight, with from about 50 toabout 200 parts being desired, based upon 100 total parts by weight ofurethane prepolymer in the formulation. The solvents can be any of thetypes utilized in the polymer component or the cure component. Thus,toluene, xylene, methyl ethyl ketone, methyl isobutyl ketone, and thelike, or combinations thereof, can be utilized.

Each fraction or component of the polyurethane formulation of thepresent invention unexpectedly has very good stability and thus goodshelf life. Thus the various separate components can be made and keptseparate for a number of months and then brought together and mixed inany conventional manner and applied to a substrate. For example, thecomponents can be fed through three separate lines to a spray gun wherethey are then mixed together and sprayed upon a desired substance orsubstrate. Generally, any conventional type of mixing device can beutilized as well as any conventional type of spraying apparatus such asany airless type of spray gun. The application of the polyurethaneformulation to the substrate can be through a spray gun, throughbrushing, coating, or the like. Upon application, the various solventsevaporate and the mixture of aromatic diamine and aliphatic diamineketimine curing agents reacts with the urethane prepolymer to yield acured polyurethane formulation. Although cure can be at ambienttemperature, that is for example at 65° F., generally heat can beapplied to speed the curing operation. Cure can thus occur from aboutambient temperature to about 180° F. and preferably from about 75° toabout 120° F. Heating at higher temperatures tends to create a porouspaint and, hence, is undesirable. Generally, any desired thickness canbe applied at one application and any number of applications can beapplied to yield a desired thickness. Usually, the final thickness canrange from about 10 to about 50 mils.

With regard to the actual mixing, the three components are generallymixed simultaneously or within a short time period of one another, asfor example a few minutes. Alternatively, the cement component can bemixed with the polymer component, but such cement-polymer mixture shouldbe combined, with the cure component generally within a few days sincestability of the cement-polymer component is not very good.

The polyurethane formulation, when prepared according to the presentprocess, has good flexibility which is retained over long periods oftime, good resistance to hydrolysis, and has excellent resistance tofuels, and the like. Moreover, the polyurethane forms a very goodsealant coat. Accordingly, it can be utilized to contain fuels as forautomobiles, aircraft, and the like. Thus, it can be sprayed as to theinside of a container, a fuel tank, a flexible rubber fuel tank, and thelike. It can also be applied to an aircraft fuselage or wing to sealsaid area. Moreover, in the wing area, a sufficient coating can be madeto form an integral fuel tank within the aircraft wing. That is, thepolyurethane of the present invention can be sprayed over variousintegral parts and thereby seal them from the fuel. Such an applicationcan result in approximately a 20 percent increase in the fuel tank area.

The following invention is further illustrated in the followingrepresentative examples.

EXAMPLE 1

The following general procedures were used to prepare the formulationsdesignated A-G in the Tables.

The urethane component was prepared by mixing the prepolymer with thesolvent in a container having an inert atmosphere therein such as anitrogen atmosphere. Upon mixture thereof, the container was sealed withthe inert gas to protect the contents from exposure to moisture. Thecement component was made by adding the various ingredients to acontainer and mixing. Desirably, the cement component was sealed in anairtight container. The ketimine of the aliphatic diamine was preparedby adding a molecular sieve drying agent to a solution of the aliphaticdiamine in an excess of a ketone solvent, usually a mixture of methylethyl ketone and methyl isobutyl ketone, the mixture was allowed tostand overnight and the molecular sieve was then removed by filtrationor decantation and the aromatic diamine was added to the resultingsolution of aliphatic diamine ketimine and the solution of the curingagents was stored in a sealed container until ready for use.

The individual components when thus prepared can be stored forrelatively long periods of time without adverse effects. Theformulations shown in the Tables were prepared by mixing the threecomponents together and immediately spraying on a second treatedcardboard surface for the purpose of preparing test samples. Portions ofthe sprayed Formulations A-G were tested as indicated in the Tables todetermine their various physical properties. Formulations A and G arefor control purposes and are outside the scope of the present invention.Formulations E and F are outside the preferred formulations of thepresent invention.

The room temperature pot life for Formulations A-F was found to be about30 minutes and the pot life for Formulation G was found to be about onehour. When the ketimine of the aliphatic diamine was not first preparedin Formulations A-F, the pot life was found to be 4 minutes or lesswhich is insufficient for most spraying or coating applications.

                                      TABLE I                                     __________________________________________________________________________                                  FORMULATION                                                                   A     B   C   D   E     F    G                  __________________________________________________________________________    Prepolymer (% NCO = 3.35)*    100.00                                                                              100.00                                                                            100.00                                                                            100.00                                                                            100.00                                                                              100.00                                                                             100.00             Toluene                       56.26 56.76                                                                             56.76                                                                             56.76                                                                             56.76 56.26                                                                              56.26              "MODAFLOW"                    1.00  1.00                                                                              1.00                                                                              1.00                                                                              1.00  1.00 1.00               "EPON 1001-B-80"              6.25  6.25                                                                              6.25                                                                              6.25                                                                              6.25  6.25 6.25               "CABOSIL M-5"                 4.57  4.57                                                                              4.57                                                                              4.57                                                                              4.57  4.57 4.57               Carbon black                  1.27  1.27                                                                              1.27                                                                              1.27                                                                              1.27  1.27 1.27               Methyl ethyl ketone           57.78 57.78                                                                             57.78                                                                             57.78                                                                             57.78 57.78                                                                              57.78              Metaphenylene diamine (MPD)   --    0.405                                                                             1.21                                                                              2.02                                                                              2.83  3.64 4.05               Methyl ethyl ketone           --    4.84                                                                              14.53                                                                             24.22                                                                             33.91 43.60                                                                              48.44              Methylene-bis(cyclohexyl amine)(H.sub.12 MDA)                                                               7.87  7.09                                                                              5.51                                                                              3.94                                                                              2.36  0.787                                                                              --                 Methyl ethyl ketone           22.31 20.08                                                                             15.62                                                                             11.15                                                                             6.69  2.23 --                 Methyl isobutyl ketone        22.31 22.08                                                                             15.62                                                                             11.15                                                                             6.69  2.23 --                 Molecular sieves, type 5A, 1/8" pellets                                                                     (1.87)**                                                                            (1.69)                                                                            (1.31)                                                                            (0.94)                                                                            (0.56)**                                                                            (0.19)                                                                             --                 TOTAL                         279.67                                                                              279.63                                                                            279.62                                                                            279.61                                                                            279.61                                                                              279.62                                                                             279.62             PERCENT SOLIDS                42.8  42.7                                                                              42.4                                                                              42.1                                                                              41.9  41.6 41.4               DIAMINE/DIISOCYANATE RATIO    0.94  0.94                                                                              0.94                                                                              0.94                                                                              0.94  0.94 0.94               AROMATIC DIAMINE/ALIPHATIC DIAMINE, WT. %                                                                    0/100                                                                              10/90                                                                             30/70                                                                             50/50                                                                             70/30 90/10                                                                              100/0              __________________________________________________________________________     *polyhexamethylene isophthalate/azelate having a molecular weight of 2,00     made with methylenebis(4-cyclohexylisocyanate).                               **molecular sieves were removed by screening just before the curing agent     blends were mixed with the other components.                             

                  TABLE II                                                        ______________________________________                                        Tensile strength                                                              (psi) Standing                                                                at Ambient FORMULATION                                                        Temperature for:                                                                         A      B      C    D     E    F    G                               ______________________________________                                        2     hours    1,000    600                                                                                226                                                                                155   150                                                                                120                                                                                120                         4     hours    1,761  1,331                                                                                722                                                                                653   600                                                                                494                                                                                588                         24    hours    2,830  2,929                                                                              2,706                                                                              2,712 2,877                                                                              2,932                                                                              2,335                         41    days     3,562  3,715                                                                              3,801                                                                              3,658 4,075                                                                              4,305                                                                              4,130                         245   days     3,295  3,660                                                                              3,777                                                                               3,801*                                                                             4,135                                                                              4,591                                                                              5,054                         ______________________________________                                         *About 4,000 psi tensile is maximum desirable for flexibility needed to       readily remove a full tank liner from the tank without causing damage to      the liner.                                                               

EXAMPLE 2

A repeat of Example 1, B-D with isophorone diamine in place of H₁₂ MDAgave similar results.

EXAMPLE 3

A repeat of Example 1, B-D using 2,3-bis(aminomethyl)cyclohexane inplace of H₁₂ MDA gave similar results.

EXAMPLE 4

A repeat of Example 1, B-D using ethylene diamine in place of H₁₂ MDAgave similar results.

EXAMPLE 5

A repeat of Example 1, B-D using metaxylene diamine in place of H₁₂ MDAgave similar results.

While in accordance with the patent statutes only the best mode andpreferred embodiment of the invention has been illustrated and describedin detail, it is to be understood that the invention is not limitedthereto or thereby, but that the scope of the invention is defined bythe appended claims.

What is claimed is:
 1. A process for preparing a urethane formulationcomprising the steps of:separately preparing a polymer component, saidpolymer component comprising a urethane prepolymer and a urethanesolvent; the amount of said urethane prepolymer in said urethanecomponent is about 100 parts by weight, wherein said urethane prepolymeris the reaction product (1) an aliphatic polyisocyanate having from 5 to25 atoms, and (2) a mixed polyester, said mixed polyester being (a) acopolymerization condensation product of, or (b) a separate physicalblend of an aliphatic dicarboxylic acid having from 2 to 12 carbon atomsand an aromatic dicarboxylic acid having from 8 to 12 carbon atoms witha polyol having a molecular weight of 500 or less, the amount of saidaliphatic dicarboxylic acid ranging from about 90 percent to about 10percent by weight, this molecular weight of said polyester made fromsaid acid and said polyol ranging from about 500 to about 4,000, andwherein the equivalent ratio of said isocyanate to the OH end groups insaid polyester ranges from about 1.5 to about 3.0; separately preparinga curing component, said curing component consisting essentially of amixture of an aromatic diamine, the ketimine of an aliphatic diamine anda curing agent solvent, the equivalent ratio of said curing agent tosaid isocyanate being from about 0.8 to about 1.2; separately preparinga cement component, said cement component comprising about 3 to about 7parts by weight per 100 parts by weight of said urethane prepolymer ofan epoxy having leaching resistance and a solvent; a thixotropiccompound, said thixotropic compound located in said cement component,the amount of said thixotropic agent being from about 2 parts by weightto about 8 parts by weight per 100 parts by weight of said urethaneprepolymer; maintaining said cement component separate from said polymercomponent and said curing agent, and maintaining said polymer componentseparate from said curing component, thereby maintaining stability ofeach of said components; the total amount of solids in said overallformulation ranging from about 20 percent to about 50 percent by weightbased upon the total amount of said solvents and said solvents in saidoverall formulation.
 2. The process of claim 1 wherein the curingcomponent is composed of an aromatic diamine which is one selected fromthe group consisting of monocyclic aryl diamines containing from 6 to 12carbon atoms and bicyclic aryl diamines containing from 12 to 20 carbonatoms and a ketimine, an aliphatic diamine selected from the groupconsisting of alkyl diamines containing from 2 to 10 carbon atoms,cycloalkyl diamines containing from 4 to 20 carbon atoms, and alkaryldiamines containing from 8 to 20 carbon atoms.
 3. The process of claim 2wherein the weight ratio of aromatic diamine to aliphatic diamine in thecuring agent is in the range of from about 10:90 to 50:50.
 4. Theprocess of claim 3 wherein the amount of said thixotropic agent rangesfrom about 2 to about 8 parts by weight and wherein the amount of saidepoxy ranges from about 3 to about 7 parts by weight.
 5. The process ofclaim 4 wherein at least 90 percent of said polyisocyanate is an alkyldiisocyanate having from 6 to 10 carbon atoms or a cycloalkyldiisocyanate having from 8 to 15 carbon atoms, wherein said polyolforming said polyester is a glycol having from 2 to 10 carbon atoms,wherein said aliphatic dicarboxylic acid has from 6 to 9 carbon atoms,and wherein the equivalent ratio of said isocyanate to the hydroxyl endgroups in said polyester ranges from about 1.8 to about 2.2.
 6. Theprocess of claim 5 wherein said polyester has a molecular weight of fromabout 1,000 to about 3,000, wherein said polyol forming said polyesterhas a molecular weight of 200 or less, wherein the amount of thixotropicagent ranges from about 3 parts to about 6 parts by weight, wherein theamount of solids in said urethane component ranges from about 50 toabout 70 percent, wherein the amount of solids in said curing agentcomponent ranges from about 5 to about 30 percent by weight, and whereinthe amount of solvent in said cement component ranges from about 25parts to about 250 parts by weight, based upon 100 parts by weight ofsaid urethane polymer or prepolymer, wherein the equivalent ratio oftotal diamine in said curing agent component to isocyanate ranges fromabout 0.9 to about 0.95.
 7. The process of claim 6 wherein saidthixotropic agent is silicon dioxide, said epoxy is4,4'-isopropylidenedipheno-epichlorohydrin and said polyol is hexanediol.
 8. The process of claim 7 wherein said polyisocyanate is methylene(bis)-4-cyclohexylisocyanate), said aliphatic dicarboxylic acid isisophthalic acid, said aromatic diamine is metaphenylene diamine andsaid aliphatic diamine is a member selected from the group consisting ofmethylene bis(cyclohexyl amine), isophorone diamine,1,3-bis-(aminomethyl)cyclohexane, ethylene diamine and metaxylenediamine.
 9. The process of claim 8 which includes mixing together saidpolymeric component, said cement component and said curing component toform a polyurethane formulation and applying said polyurethaneformulation to a substrate and curing said formulation at from ambienttemperature to about 180° F.
 10. The process of claim 9 wherein saidsubstrate is an aircraft structure, a fuel tank or a flexible fuelcontainer.
 11. A process according to claim 9 wherein the substrate isan aircraft wing component.
 12. A polyurethane, said polyurethane madeby separately preparing a polymer component, a curing component and acement component;said polymer component comprising a urethane prepolymerand a urethane solvent, said curing component consisting essentially ofa curing agent which is a mixture of an aromatic diamine, the ketimineof an aliphatic diamine and a curing agent solvent, said cementcomponent comprising from about 3 to about 7 parts by weight per 100parts by weight of said urethane prepolymer, an epoxy having solventleaching resistance, and a solvent; the amount of said urethaneprepolymer in said urethane component is about 100 parts by weight,wherein said urethane prepolymer is the reaction product of (1) analiphatic diisocyanate having from 3 to 12 carbon atoms, or an alicyclicpolyisocyanate having from 5 to 25 carbon atoms, (2) a mixed polyester,said mixed polyester being (a) a copolymerization condensation productof, or (b) a separate physical blend of, an aliphatic dicarboxylic acidhaving from 2 to 12 carbon atoms and an aromatic dicarboxylic acidhaving from 8 to 12 carbon atoms with a polyol having a molecular weightof 500 or less, the amount of said aliphatic dicarboxylic acid rangingfrom about 90 percent to about 10 percent by weight, the molecularweight of said polyester made from said acid and said polyol rangingfrom about 500 to about 4,000 and wherein the equivalent ratio of saidisocyanate to the OH end groups in said polyester ranges from about 1.5to about 3.0; the equivalent ratio of said curing agent to saidisocyanate being from about 0.8 to about 1.2; a thixotropic compoundlocated in said cement component, the amount of said thixotropic agentbeing from about 2 parts by weight to 8 parts by weight per 100 parts byweight of said urethane prepolymer; said urethane being produced bymaintaining said polymer component, said curing component, and saidcement component separate from each other until prior to application,maintaining each said component separate from each other and therebymaintaining the stability of each of said components and subsequentlymixing said three components together and curing at from about ambienttemperature to about 180° F.
 13. A polyurethane according to claim 12wherein the curing agent is composed of an aromatic diamine which is oneselected from the group consisting of monocyclic aryl diaminescontaining from 6 to 12 carbon atoms and bicyclic aryl diaminescontaining from 12 to 20 carbon atoms and a ketimine of an aliphaticdiamine selected from the group consisting of alkyl diamines containingfrom 2 to 10 carbon atoms, cycloalkyl diamines containing from 4 to 20carbon atoms, and alkaryl diamines containing from 8 to 20 carbon atoms.14. The polyurethane of claim 13 wherein the weight ratio of aromaticdiamine to aliphatic diamine in the curing agent is in the range of fromabout 10:90 to 50:50.